The Impact of Nonequilibrium Flow on the Structure of Turbulence Over River Dunes

Abstract

Most past experimental investigations of flow over river dunes have focused on conditions that match semiempirical flow‐depth scaling laws, yet such equilibrium conditions are of limited value because they rarely occur in natural channels. This paper quantifies the structure of mean and turbulent flow over fixed 2‐D laboratory dunes across a range of nonequilibrium conditions within the dune flow regime. The flow field was quantified using 2‐D particle imaging velocimetry for 12 conditions, including flows that are too deep, too shallow, too fast, or too slow for the size of the fixed dunes. The results demonstrate major departures in the patterns of the mean flow and structure of turbulence when compared to dunes formed under equilibrium flow conditions. The length of flow reattachment scales linearly with the ratio of mean depth‐averaged streamwise velocity to shear velocity at the dune crest ( italicUtrue¯c/uc*), which provides a new predictive measure for flow reattachment length. Depth‐averaged vertical velocities at the dune crest ( Vtrue¯c) show a parabolic relationship with Utrue¯c, peaking at Utrue¯c ~0.60 m/s, which matches the relationship of dune aspect ratio with transport stage present in mobile bed conditions. The spatial location of the turbulent wake was found to vary with flow depth and velocity, with lower Utrue¯c and greater flow depths causing the wake to rise toward the free surface. Deeper flows are likely to show less flow convergence over the crests of dunes due to reduced interaction of turbulence with the free surface, resulting in a reduction of transport stage.

Scale‐Dependent Evanescence of River Dunes During Discharge Extremes

“…Dune shape and slipface angle control flow resistance and sediment transport conditions by setting the flow structure behind dunes. Initially, at BLD transport conditions (EXP1 to EXP3), dunes were triangular due to low topographic acceleration and minimal crestal flattening, reflecting bed load avalanching process at dune leesides (Best, 1996; Carling et al, 2000; Unsworth et al, 2018). At slightly higher sediment transport conditions (EXP4), the largest dunes were produced due to deposition at the dune crest promoted by high vertical velocities, and increased trough scour associated with larger Reynold stresses at the flow reattachment point (Table S1).…”

“…At slightly higher sediment transport conditions (EXP4), the largest dunes were produced due to deposition at the dune crest promoted by high vertical velocities, and increased trough scour associated with larger Reynold stresses at the flow reattachment point (Table S1). Dunes with long flat crests, often referred to as humpback dunes, linked to increased crestal flattening (Bridge & Best, 1988; Saunderson & Lockett, 1983; Unsworth et al, 2018), appear to dominate the flume bed at higher transport conditions (EXP 5 to EXP9).…”

“…Factors widely considered in literature are (i) increased transport of bed material into suspension, (ii) deposition of suspended sediment in dune troughs, and (iii) bedform superimposition and amalgamation (Best & Fielding, 2019). Strong free surface interaction with the bed in shallow laboratory flows on the other hand is considered to be a contributory factor in producing HADs in shallow flows (Andreotti & Claudin, 2013; Fourrière et al, 2010; Naqshband et al, 2014; Unsworth et al, 2018). Recent advances further highlight the potential of maintaining high‐angle slipfaces on HADs by granular avalanches, whereas lower slipface angles of LADs have been interpreted as the product of loosely packed liquefied avalanches amplified by downslope currents (Kostaschuk & Venditti, 2019).…”

Shoaling of an Internal Wave Packet in an almost Three-Layer Sea over a Steep Shelf

“…To provide a range of realistic bed states to test these ideas on, Multibeam echosounder (MBES) bathymetry from the Mekong River in Cambodia is used as it provides a range of flow discharges, bedform states and is large enough that superimposed bedforms are well resolved in the bathymetry. (Unsworth et al, 2018); b) half-sine wave shape is from (Nelson et al, 1993). The repeating dune shapes produce a sine-wave in the semi-variogram that is correlated to the dune troughs -where the profiles begin.…”

The Influence of Slipface Angle on Fluvial Dune Growth

“…Understanding dune growth requires consideration of turbulent flow fields, dune morphology, sediment transport, and the redistribution of sediment over and among dunes, details of which are not captured by mean geometric parameters such as dune height and length (Parsons & Best, 2013;Reesink et al, 2018). Significant progress has been made into understanding the dynamics of flow separation and eddy generation over fixed and mobile, high-angle dunes (HADs, ∼30°) (e.g., Bourgoin et al, 2020;Dey et al, 2020;Naqshband et al, 2014cNaqshband et al, , 2017Omidyeganeh & Piomelli, 2013) and low-angle dunes (LADs, <10°) (e.g., Best & Kostaschuk, 2002;Kwoll et al, 2017;Lefebvre et al, , 2019Motamedi et al, 2012Motamedi et al, , 2014Unsworth et al, 2018). Yet quantitative observations and simulations of sediment fluxes along migrating and changing dune forms remain extremely rare (Naqshband et al, 2014b(Naqshband et al, , 2014c(Naqshband et al, , 2017.…”